1. Field of Invention
The present invention relates to a process for abating pollution, and more particularly, to an efficient process for substantially completely burning combustibles such as carbon monoxide in waste gas to meet or exceed the air pollution control regulations pertaining to allowable contaminants in gases exhausted to the atmosphere.
2. Description of the Prior Art
Most waste gases that evolve from processes involving the combustion of carbonaceous fuels include certain variable amounts of carbon monoxide. For example, the waste gas evolving from the production of aluminum chloride by chlorination of porous alumina intermixed with carbon, as disclosed in Russell et al. U.S. Pat. No. 3,842,163, includes approximately 50-75% nitrogen, 20-40% carbon dioxide, 0-35% carbon monoxide, with the balance being water vapor. Those skilled in the art will realize that the water vapor is actually picked up as evaporation of recycle water utilized in the gas scrubbers. For our purposes, the waste gas system is considered to be included in the production of aluminum chloride (Al.sub.2 Cl.sub.6). Current air pollution regulations limit the amount of carbon monoxide that can be emitted to the atmosphere in exhaust gas. Such exhaust gases must usually be approximately 99.5% carbon monoxide free to meet or exceed the regulations.
In order to meet such regulations the waste gas may be incinerated in a combustion chamber maintained at a temperature above the combustion temperature of carbon monoxide (approximately 1157.degree. F [625.degree. C]) and supplied with sufficient excess secondary air to insure that the carbon monoxide is converted into carbon dioxide. The prior art, such as Lewis et al. U.S. Pat. No. 3,049,300, discloses the desirability of providing an optimum amount of excess air to avoid unnecessary heat loss and unburned fuel loss in combustion processes operated with a multiplicity of fuels for the purpose of evolving heat. It is important to maintain the combustion chamber temperature in a range of approximately 1200.degree. F (650.degree. C) to 2000.degree. F (1100.degree. C). If the temperature falls below 1157.degree. F the carbon monoxide cannot burn, and if the temperature substantially exceeds 1157.degree. F the heat tends to adversely affects the life of the refractory lining in the chamber.
The present invention is directed to a process for the conversion of carbon monoxide in waste gas into carbon dioxide when both the volume of waste gas and the percentage concentration of carbon monoxide are continuously fluctuating. Such fluctuations in volume and in percent carbon monoxide concentration make control of the process difficult. To insure that the exhaust gas from the combustion chamber continuously meets the air pollution regulations, the prior art suggests that combustion of the gas should be based on assumed worst case conditions. Worst case conditions are those in which the quantity of carbon monoxide in the waste gas is at a maximum expected under normal operation conditions.
Once a determination has been made as to the maximum practicable amount of carbon monoxide that can be expected to be fed into the combustion chamber under worst case conditions, sufficient secondary air can be supplied to the combustion chamber. The secondary air is required for complete combustion of the maximum quantity of carbon monoxide to insure continuous complete combustion. Sufficient fuel can also be supplied to the combustion chamber to maintain a relatively constant temperature in the combustion chamber to insure stable and complete combustion of the carbon monoxide.
Conversion of carbon monoxide to carbon dioxide based on an assumed worst case condition is inefficient. Secondary air is supplied to the combustion chamber in an amount necessary to convert the maximum practicable quantity of carbon monoxide into carbon dioxide. For the substantial majority of the time, less than the maximum quantity of carbon monoxide is in the waste gas. Therefore, for the substantial majority of the time an overabundance of secondary air is being supplied to the combustion chamber. Excess air has the effect of cooling the combustion chamber. When the chamber is unnecessarily cooled, more fuel must be supplied to the chamber to maintain the necessary combustion temperature. This results in a significant waste of fuels, such as natural gas, coal, coke, blast furnace gas, mixed gas, etc.
The way to eliminate the waste of combustion fuel is to operate the combustion chamber under actual case conditions rather than worst case conditions. The problem with operating under actual case conditions has been that the expense involved in installing, operating and maintaining the sensors, meters and other devices for continuously measuring the fluctuating quantity of carbon monoxide in the fluctuating volume of waste gas far outweights the potential fuel savings.
Accordingly, an economical and effective process is desired for efficiently converting carbon monoxide in waste gas into carbon dioxide in a combustion chamber.